This invention relates to hydrophilic, humectant, soft, pliable, absorbent paper having wet strength agents and a method for its manufacture. The absorbent paper products of this invention such as napkins, bathroom tissue, facial tissue, and towels are exceedingly soft to the touch yet strong enough to withstand vigorous use. The pleasingly soft touch to the human skin is achieved by the use of cationic softeners having humectancy properties and also melting points in the range of about 0.degree. to 40.degree. C. Cationic softeners which exhibit humectancy properties and are liquid at ambient temperatures produce a hydrophilic, humectant, soft, absorbent paper product. The usual cationic softeners do not exhibit humectancy properties and have much higher melting points and therefore do not impart the soft, hydrophilic, humectant properties to the absorbent paper.
In general, the prior art method of imparting softness to cellulosic tissue paper sheets is to apply work to the sheets. For example, at the end of most conventional tissue papermaking processes, the sheets are removed from the surface of a thermal drying means, such as a Yankee drum, by creping them with a doctor blade. Such creping breaks many of the inter-fiber hydrogen bonds throughout the entire thickness of the sheet. However, such simple creping produces tissue paper that is neither as soft nor as strong as is desirable.
The prior art therefore turned to treating cellulosic tissue paper sheets or their cellulosic web precursor, with chemical debonding agents that disrupt the inter-fiber hydrogen bonds. See, e.g., U.S. Pat. Nos. 4,144,122; 4,372,815; and 4,432,833.
For example, U.S. Pat. Nos. 3,812,000; 3,844,880; and 3,903,342 disclose the addition of chemical debonding agents to an aqueous slurry of cellulosic fibers. Generally, these agents are cationic quaternary amines such as those described in U.S. Pat. Nos. 3,554,863 and 3,395,708. Other references disclose adding the chemical debonding agent to a wet cellulosic web. See, U.S. Pat. No. 2,756,647 and Canadian Patent No. 1,159,694. These prior art methods have been found to produce hydrophobic paper products which are not comparable to the hydrophilic, humectant, soft, pliable, absorbent paper product of this invention.
Paper webs or sheets find extensive use in modern society. These include such staple items as paper towels, facial tissues, sanitary (or toilet) tissues, and napkins. These paper products can have various desirable properties, including wet and dry tensile strength, absorbency for aqueous fluids (e.g., wettability), low lint properties, desirable bulk, and softness. The particular challenge in papermaking has been to appropriately balance these various properties to provide superior absorbent paper.
Although desirable for towel products, softness is a particularly important property for facial and toilet tissues and napkins. Softness is the tactile sensation perceived by the consumer who holds a particular paper product, rubs it across the skin, and crumples it within the hand. Such tactile perceivable softness can be characterized by, but is not limited to, friction, flexibility, and smoothness, as well as subjective descriptors, such as a feeling like velvet, silk, or flannel. This tactile sensation is a combination of several physical properties, including the flexibility or stiffness of the sheet of paper, as well as the texture of the surface of the paper.
Wet strength is enhanced by the inclusion of certain wet strength resins, that, being typically cationic, are easily deposited on and retained by the anionic carboxyl groups of the papermaking fibers. However, the use of chemical means to improve dry and wet tensile strength can also result in stiffer, harsher feeling, less soft, absorbent papers. This, however, is not the case for our products which contain cationic softeners. We add about 1 to 30 pounds of the wet strength resin per ton of furnish, preferably 2 to 10 pounds for bathroom and facial tissue and napkin, and preferably 5 to 20 pounds for towel. The suitable range for bathroom tissue is 1 to 20 pounds while for towel it is 1 to 30 pounds.
Certain chemical additives, commonly referred to as debonding agents, can be added to papermaking fibers to interfere with the natural fiber-to-fiber bonding that occurs during sheet formation and drying, and thus lead to softer papers. These debonding agents have certain disadvantages associated with their use in softening absorbent papers. Some low molecular weight debonding agents can cause excessive irritation upon contact with human skin. Higher molecular weight debonding agents can be more difficult to apply at low levels to absorbent paper and also tend to have undesirable hydrophobic effects on the absorbent paper, e.g., result in decreased absorbency and particularly wettability. Since these debonding agents operate by disrupting inter-fiber bonding, they can also decrease tensile strength to such an extent that resins, latex, or other dry strength additives can be required to provide acceptable levels of tensile strength. These dry strength additives not only increase the cost of the absorbent paper but can also have other, deleterious effects on absorbent paper softness.
Debonders serve to make a softer sheet by virtue of the fatty portion of the molecule which interferes with the normal hydrogen bonding. The use of a debonder can reduce the energy required to produce a fluff to half or even less than that required for a nontreated pulp. This advantage is not obtained without a price, however. Many debonders tend to reduce water absorbency as a result of hydrophobicity caused by the same fatty long chain portion which gives the product its effectiveness. Those interested in the chemistry of the debonders will find them widely described in the patent literature. The following list of U.S. patents provides a fair sampling, although it is not intended to be exhaustive: Hervey et al., U.S. Pat. Nos. 3,395,708 and 3,554,862; Forssblad et al., U.S. Pat. No. 3,677,886; Emanuelsson et al., U.S. Pat. No. 4,144,122; Osborne, III, U.S. Pat. No. 4,351,699; and Hellsten et al., U.S. Pat. No. 4,476,323. All of the aforementioned patents describe cationic debonders. Laursen, in U.S. Pat. No. 4,303,471, describes what might be considered a representative nonionic debonder.
U.S. Pat. No. 3,844,880 to Meisel, Jr., et al. describes the use of a deposition aid (generally cationic), an anionic resin emulsion, and a softening agent which are added sequentially to a pulp furnish to produce a soft product having high wet and dry tensile strength. The opposite situation; i.e., low wet tensile strength, is preferred for a pulp which is to be later reslurried for some other use.
Croon et al, in U.S. Pat. No. 3,700,549, describe a cellulosic fiber product crosslinked with a polyhalide, polyepoxide, or epoxyhalide under strongly alkaline conditions. All of the crosslinking materials are insoluble in water. Croon et al. teach three methods to overcome this problem. The first is the use of vigorous agitation to maintain the crosslinking agent in a fine droplet-size suspension. Second is the use of a polar cosolvent such as acetone or dialkylsulfoxides. Third is the use of a neutral (in terms of being a nonreactant) water soluble salt such as magnesium chloride. In a variation of the first method, a surfactant may be added to enhance the dispersion of the reactant phase. After reaction, the resulting product must be exhaustively washed to remove the necessary high concentration of alkali and any unrelated crosslinking material, salts, or solvents. The method is suitable only for cellulosic products having a relatively high hemicellulose content. A serious deficiency is the need for subsequent disposal of the toxic materials washed from the reacted product. The Croon et al. material would also be expected to have all other well known disadvantages incurred with trying to use a stiff, brittle crosslinked fiber.